1. Technical Field
The present invention relates to a microporous polyolefin multilayer film stable in quality and outstanding in thermal stability. More particularly, the present invention relates to a microporous film that has a feature to have both a low shutdown temperature caused by polyethylene and a high melt fracture temperature caused by polypropylene and a heat-resistant filler, as well as both uniform micropores and high strength/stability characterized in the separator prepared under wet process and high permeability and high strength characterized in the macropores prepared under dry process. Therefore, this multi-layered film can be used effectively to manufacture a secondary battery with a high capacity and high power.
2. Description of the Related Art
The microporous polyolefin film is widely used to manufacture battery separators, separating filters, membranes for microfiltration, and the like, because it is chemically stable and outstanding in physical properties.
The method for preparing a microporous film by using polyolefin is often accomplished under wet process. This procedure is comprised of the steps: mixing polyolefin and a diluent at high temperature to make a single phase; cooling to separate the polyolefin phase and the diluent phase; and then, extracting the diluent to generate pores in the polyolefin. The procedure can be applied to manufacture a thin film so that it is popularly used for high-capacity, high-power lithium ion secondary battery, and the like, due to the high strength and permeability, uniform pores and the evenness of quality.
General method for preparing the porous film by using wet process has been disclosed in U.S. Pat. No. 4,247,498. This method described the steps: selecting a diluent proper polyethylene; blending the mixture at high temperature to make a thermodynamically single-phase solution; and cooling to separate the polyolefin phase and the diluent phase so as to fabricate a porous film of the polyolefin.
The lithium ion secondary battery is an outstanding battery very high in the energy density, but is associated with the risk of explosion caused by short circuit. Therefore, a high level of quality is required for the separator with the evenness of quality. Further, as the application of the lithium ion secondary batteries is extended to hybrid cars and other fields, a stricter thermal stability requirement is required for the separator. When the thermal stability decreases, the battery may explode dangerously due to rupture separator when being overheated. It is because poor thermal stability of the separator may lead to overheating of the battery and explosion caused by melting and rupturing separators.
Thermal stability of separator in a battery is determined by shutdown temperature and melt down temperature.
Shutdown temperature is the temperature at which the micropores of the separator are closed to shut the electric current when the inside of the battery is abnormally overheated. Melt down temperature is the temperature at which the separator is subjected to melt and the electric current flows again when the battery temperature increases beyond the shutdown temperature. To ensure thermal stability of a battery, it is preferred that the shutdown temperature is low and the melt down temperature is high. Especially, the melt down temperature is closely related with the battery stability, because it shuts electric current continuously even under an explosive situation.
In order to improve the thermal stability of the separator, three kinds of approaches have been attempted. One is to add an inorganic material or heat-resistant resin to the polyethylene in order to increase the thermal stability of the separator. Another is to coat a heat resistant material on the surface. The other is to manufacture a multi-layered separator containing a heat resistant layer.
U.S. Pat. No. 6,949,315 discloses a method blending UHMW (ultra high molecular weight) polyethylene with 5-15 weight % of an inorganic material such as titanium oxide to improve thermal stability of the separator. However, this method is disadvantageous in that, although the addition of the inorganic material provides the effect of improving thermal stability, it may lead to poor mixing and nonuniform quality and formation of pinholes resulting therefrom, or poor film properties because of lack of compatibility at the interface between the inorganic material and the polymer resin. These disadvantages are inevitable in a separator using an inorganic material.
U.S. Pat. No. 5,641,565 discloses a method of blending a resin having superior heat resistance instead of using an inorganic material. In this method, the separator is manufactured by the steps: mixing polyethylene and 5 to 45 weight % polypropylene to make a resin mixture; mixing 30 to 75 weight % of an organic liquid material and 10 to 50 weight % of an inorganic material; and extracting the organic liquid material and the inorganic material. However, there still remains the problem associated with mixing the inorganic material as described above, even though the inorganic compound is extracted out. As already mentioned in the patent document, the addition of polypropylene which is not compatible with polyethylene leads to deterioration of physical properties. Further, this procedure is complicated due to the additional steps of extracting and removing the inorganic compound. It requires a lot of polypropylene in order to improve thermal stability and, thus, further deteriorates the physical property of the separator.
The method for coating a heat resistant material on the surface of a microporous film is disclosed in U.S. Patent Application No. 2006-0055075A1. Unfortunately, the coating system is restricted in increasing the permeability of the coating layer. As a consequence, the film has poor permeability and the wetting property between the coating layer and the microporous film is likely to be poor to cause unevenness of quality.
In order to improve the thermal stability of the separator, lamination is used to prepare a multi-layered separator. In U.S. Pat. No. 5,691,077, polyethylene outstanding in the shutdown property (i.e., low melting point) is laminated with polypropylene having a high melt down temperature (i.e., high melting point) to fabricate a 3-layer-structured separator. This separator is outstanding in the thermal property, but has a lot of disadvantages. In detail, this laminated separator is associated with nonuniform stretching, pinhole generation, increased deviation of thickness, and the like, because films are prepared through a low-temperature dry process. It lowers productivity because of the additional laminating step. Further, the separator may be delaminated due to poor adhesion strength. As a consequence, this method is not applied widely. In spite of superior thermal stability, this method does not satisfy the necessary requirements for the separator of a secondary battery, such as strength, permeability, uniform quality and productivity.
In Japanese Patent Laid-Open No. 2002-321323 and WO 2004/089627, the multi-layered separators that comprise a polyethylene microporous film prepared under wet process as a main layer and layer comprised of a mixture of polyethylene and polypropylene also prepared under a moisturized condition wet process as a surface layer, have been disclosed. These separators are outstanding in the quality stability, because they are manufactured under wet process, but are restricted in that they do not have a thermal stability better than that of the polypropylene resin. Furthermore, the processes for preparing a multi-layered separator become complicated because all the layers of the separator should be manufactured under wet process.
In WO 2006/038532, the multi-layered separator containing inorganic particles prepared under wet process has been introduced. This separator should be fabricated under wet process, as described above. In addition to a complicated blending process, this method is associated with the problem of deteriorated physical properties caused by the inclusion of the diluent, which has to be extracted out during film production, by more than 50% on the surface layer (Stretching performed in the presence of a diluent does not give desired effect).
In the separator of a secondary battery, the characteristics of strength, permeability and evenness of quality are essential, and, recently, superior thermal stability is also required. However, the conventional methods mentioned above cannot accomplish the requirements of strength, permeability, thermal stability and evenness of quality simultaneously to the level of the separator prepared under wet process.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.